Systematic investigations of nanostructuring by scanning tunneling microscopy

Scanning tunneling microscopes allow the formation of structures, by the application of voltage pulses between tip and sample whose dimensions are in the range of some nanometers. Systematic investigations have been carried out on the Si(111)-7x7 using W and Au tips in order to better understand the physics of the underlying deposition process. Therefore the voltage pulse, current, and z-piezo voltage were measured as a function of time. Above a threshold voltage, which depends on the pulse duration and the tunneling current, too, hills were created in the range between 5 and 25 nm. The surface structure was preserved up to the hills. Further, there is a linear dependence of the diameter on the pulse amplitude and pulse duration. A fit of the data gives a slope of (0.1+/-0.02) nm/s for tungsten tips and (0.45+/-0.14 nm/s) for gold tips, respectively. In addition, there is also a logarithmically dependence of the diameter upon the tunneling current for both tip materials. The results obtained are discussed with reference to the formation mechanisms published to date. None provide a satisfying explanation of our results. (C) 1996 American Vacuum Society.